Elite Four

In Pokemon Red & BlueGameboy games (and Pokemon Stadium on N64, apparently), the last challenge before the epic, mightiest Final Battle against the reigning Pokemon Master is the series of battles against the Elite Four.

The battles against them will be pretty interesting, because there's no chance for you to go back to the Pokemon Center to heal once you've started! This means you probably want to bring a big bunch of Potions, Revives and Elixirs/Ethers with you...